What do you do when your Freedom of Information request is
completed and when you read the files you found remarks or
information on a file which were not correct?

You are allowed request the FOI officer who supplied you the
file make notations on the file that the information it contains
is not correct.

(You would need to discuss this matter with the FOI officer
about how you go about this type of correction of facts. The file
cannot be changes or altered. But a notation can be added to show
what the information may not be correct.)